Distance relays are used primarily to detect underimpedance conditions on a power transmission line, due to a fault or other condition on the line, although they may be used in other applications as well. One particular type of distance relay is known as the "mho" relay. The mho relay has a circular impedance plane characteristic and in operation, discriminates between transmission line impedances which are inside or outside of the mho circle. An actual impedance for a particular line portion which is determined by the relay to be inside the mho circle is an indication of an underimpedance condition on that particular portion of the line, which in turn is a reliable indication that there is a fault on that portion of the line.
While the mho circle distance relay is reliable and has been widely used for several decades, a large number of individual mho elements are required to cover all of the fault types in a three phase line. When four or even more zones along a given portion of transmission line are covered by a conventional distance relay, twenty-four or even more mho elements may be necessary. Associated computation/comparison elements and computation time are necessary for the operation of each mho element, to calculate the impedance condition. Thus, for a distance relay covering a number of overlapping zones, including all fault types, a large number of elements is required and the overall system is computationally intensive.
From the standpoint of system cost, as well as reliability and efficiency, it would be desirable to reduce the number of mho elements and the computational burden in such a relay without decreasing the protective capability of the relay.